One of the traditional forms for the first or last movement of a symphony
is the Sonata-Allegro form.  It has three large sections, first exposition
where three themes are introduced, a first theme in the tonic key, a second
theme in a contrasting key (usually dominant) and a closing theme ( usually
tonic).  The second section is named development, where the themes are
developed by the composer.  The final section is titled recapitualation.
The themes are restated and the movement closes with the closing theme.
When I taught classroom music, Beethoven's Fifth, first movement, was my
favorite example to let the kids hear the themes as they are easily
recognized.  Another traditional form is the rondo.  It is a theme (A) then
another theme (B) the first (A) another theme (C) the first (A).  Some
composers write new themes to alternate all the way through and some write
a symmetrical form.  ABACADACABA as contrasted to ABACADAEAFAGA.  The key
of the different sections is up to the composer.  The typical four movement
symphony is usually 1st mvmt fast, 2nd mvmt slow, 3rd mvmt minuet or
scherzo, 4th fast.  The movements are usually centered around one tonality,
with all movements in a symphony being related ie 1st in Cmaj, 2nd in Fmaj,
third in Gmaj and 4th in C again.  Again the choice is up to the composer,
but usually the tonalities are related among the circle of fifths.  Another
unilfying feature sometimes used is cyclical treatment.  Themes from
previous movements show up later in the symphony.  My favorite for teaching
is Dvorak's Ninth "From the New "World".  In the last movement, students
can pick out themes from the first three movements easily.  The above is
just an off the cuff simplified effort to help clarify some thoughts I had
to pass on.  No intent to insult anyone, my foote has 5 little piggies just
like everyone else's.

James M. Arnold